I keep seeing people using the terms "dB", "dBm", and "dBi" interchangeably, when they actually mean very different things. So, here's a little background on the correct usage of the terms. Sorry if this is covered in the FAQs and links, but from the posts here, I don't think people have read them thoroughly.

A dB is a RELATIVE measure of two different POWER levels. There's also dB relative to VOLTAGE levels, but I won't go into those, as we're mostly concerned with POWER levels in our discussions here. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). Thus, I can say that an amplifier has 30 dB of gain, or I have 6dB total feedline loss. I CANNOT say, My amp puts out 30 dB, or I have a 24dB antenna, as you must state what you're referencing it to, which is where the subscript comes in. The dB by itself is not an absolute number, but a ratio.

For amplifiers, a common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt. How much gain it has is a different matter entirely, and you can have two different amps, each with an output of 30dBm (1Watt), that have different gains, and require different levels of drive power to achieve their outputs. You can also have two different amps with the same gain that have different output powers.

There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with Big Stuff, as 30dBW is 1000w, and way beyond what we deal with here!

For antennas, a common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source. An Isotropic source is the perfect omnidirectional radiator, a true Point Source, and does not exist in nature. It's useful for comparing antennas, as since its theoretical, its always the same. It's also 2.41 dB BIGGER than the next common unit of antenna gain, the dBd, and makes your antennas sound better in advertising. The dBd is the amount of gain an antenna has referenced to a DIPOLE antenna. A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd, since we're comparing it to itself. If I say I have a 24dB antenna, it means nothing, as I haven't told you what I referenced it to. It could be a 26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) antenna, depending on what my original reference was. The difference is 4.81dB, a significant amount. Most antenna manufacturers have gotten away from playing this game, but the reference will be different in different fields.

Commercial antennas tend to be rated in dBi, as the people buying them understand it, and Amateur Radio antennas tend to be dBd, as Hams are very familiar with dipoles. Sorry to go on for so long, but as an Engineer, it bugs me a bit to see things like this!

Thank you to drjim for this information.

Feedback received on this FAQ entry:

There is one little, but important, thing to mention here: dBi states the gain of an antenna in it's favored direction related to isotropic antenna (at point located the same distance away). (so, ideal dipole antenna would have zero gain over isotropic antenna in dipole axis direction :)

2014-10-29 05:25:14

For the true math geeks, the formula given could cause confusion. It should read something like dB = 10*LOG(P2/P1). Be sure to use a 10base log!

2014-09-17 11:23:33

It's such a shame that after all your great explanation about dBm and dBi you still get questions like "which antenna is better, one rated at 44 dBm or 44 dBi. Folks, wake up and read what he said. He said in the explanation dBi is gain in relation to a perfect isotropic antenna and 44 dBm is a power level and has nothing to do with gain. My guess is the 44 dBm rating is how much transmit power can be used with it (25 watts).

2014-09-16 12:55:37

Thankyou! Very helpful guide! :D

2014-05-18 22:10:23

Thanks , clarified

2014-05-05 09:55:32

is this a typo? - "It could be a 26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) "
Otherwise a great read, thanks for taking the time to share your rant.

2014-04-08 20:03:20

Thanks, very useful.

2013-12-09 05:11:14

Thanks , very helpful !

2013-11-26 15:55:23

Thanks for noble efforts.

2013-09-11 04:36:59

Thank you for this very useful clarification. And it is well explained :)

2013-08-27 16:36:05

I AM LOOKING TO BUY A 44 DBM ANTENNA FOR MY WIFI USE...WHAT DO YOU THINK? ITS MOUNTED OUTSIDE AND ITS A LINE OF SIGHT ANTENNA NOT UNIDIRECTIONAL.ID LIKE TO GET SOME FREE WIFI.

2013-05-26 21:31:32

Thank you so much... I got the difference.....

2013-05-14 05:08:14

why dbm is negative when it is raaceived by mobile

2013-02-26 12:34:56

Clearly explained..Thanks

2013-01-25 10:33:57

thank u so much for this good explanation...its cleared many of my doubts...

2012-12-11 09:05:14

Thanks a lot. It's very useful.

2012-10-30 01:42:04

Thank you very much for being so tenuous.

2012-03-07 02:14:02

Thanks for this noble effort to clearly explain the terms.RaNa AsIm

2011-11-14 10:29:50

Thanks!
I am about 300 meters LOS from my WI-FI source and was hoping to improve the quality of the reception.
The reason I'm here is because I've seen one WI-FI antenna being rated at 44dBi and another one, rated at 44dBm.
While the specs appeared similar, there was a marked difference in the price.
So ... I "Goggled" to find out.
While I'm still in the dark as far as choosing between the antennas, I have reached the conclusion that I'm the best I'll ever be in reference to myself.
Refresing!
Jerry

2011-05-07 16:01:36

I asked my friend how is the received power at the terminal (say a cell phone) the same with or without using HPA whcih causes high gain at the transmitter output?...he said received input might be same but you have high gain..how is tyhat possible?

2011-02-04 22:59:10

Thank you very much, it help me and cleared my points....
Awais from Pakistan

2011-01-13 19:52:14

It is true that isotropic point sources do not exist in nature. However, it is possible to design an antenna with an almost isotropic radiation pattern. I used the trial version of this software www.antennasoftware.com.ar to model the isotropic antenna using wires.

2010-12-25 16:06:24 (kekosat300)

Thanks..Useful info however need some more clarity over the same in case of addition of db & dbm

2010-12-22 00:17:54

thanq 4 dis useful information

2010-10-19 13:08:42

Thanks for this noble effort to clearly explain the terms.

2010-05-26 09:32:40

thanks for such a useful explanation.....
now every thing is clear to me....
pramod

2010-05-15 03:15:21

This is a fine article, and a valid peeve.
Please check the numbers in the "24 dB antenna" discussion - I think the correct presentation is:
"It could be a 24dBd antenna (26.41dBi), or a 24dBi antenna ( 21.59dBd), depending on what the original reference was, isotropic or dipole. The difference is 2.41dB, a significant amount" -- Mike

2010-04-07 12:51:29

Nice description!

2010-03-21 15:51:02

Great article! As a ham I always enjoy reading useful information like this ...Thanks..